The formation of stones in a kidney is a common pathological condition in humans. There exist a variety of kidney stones, according to their chemical composition, the most common consisting of calcium oxalate,.calcium phosphate and magnesium ammonium phosphate. It is important to find out whether a person has an inclination to form kidney stones, and of which type. This has been determined up to now mainly after surgery by analysis of the removed stones. In U.S. Pat. No. 4,399,003 there is described a method for diagnosing a patient's proneness to develop calcium oxalate kidney stones, which is based on an assay involving the urine of the patient and calcium and oxalate ions. The measurement is made by means of a calcium ion specific electrode. The present invention relates to an improvement of the said method.